(a) Field of the invention
The present invention relates to a thermosensitive recording material, in particular relates to a thermosensitive recording material improved so as to attain a high density developed color image by employing a colorless or light-colored leuco-dye and a specified compound (developer) as color forming components and further adding a specified thermosensitivity promoter thereto.
(b) Description of the prior art
The conventional thermosensitive recording material normally comprises forming the so-called thermosensitive color forming layer devised so as to form color when heated on the surface of a substrate such as paper, plastic sheet, synthetic paper or the like, wherein a thermal printer housing a thermal head therein or the like is utilized for heating purposes. The thermosensitive recording process using such a recording material has hitherto been utilized for the output recording purposes in electronic calculators, facsimile, measuring machines and the like, to say nothing of having been utilized for the reproduction purpose of books, documents and the like. In particular, the recording material, prepared by forming a thermosensitive color forming layer containing a colorless or light-colored leuco-dye with a lactone, lactam or spiropyran ring and an acidic substance (for instance, an organic acid or a phenolic substance) as thermosensitive color forming components on a substrate, has been publicly utilized because it produces a clear-cut color tone.
However, lately there have been increasing requirements for high speed and high density in said output machines, and the fact is that the thermosensitive recording material utilizing the above mentioned color forming components can not fulfil such requirements.
To cope with such requirements, there have usually been made various attempts to promote the thermosensitivity in thermosensitive recording and to achieve high speed in recording by incorporating thermosensitive promoters such as stearic acid amide, linolic acid amide, lauric acid amide, myristic acid amide, hardened beef fatty acid amide, palmitic acid amide, oleic acid amide, rice sugar fatty acid amide, coconut fatty acid amide, or methylol compounds of these fatty acid amides, and fatty acid amide type compounds such as methylenebisstearic acid amide, ethylenebisstearic acid amide and the like in the thermosensitive color forming layer. However, such thermosensitive recording materials can not meet the requirement for thermosensitivity satisfactorily and are noted defective in that the use of said thermosensitive promoters in large quantities causes attachment of dregs to the thermal head during long-run recording, resulting in a sticking phenomenon which acts to retard the sliding speed of the head and deteriorate the clearness of the developed color image heavily, whereby it becomes necessary to clean the thermal head so often in order to remove such trouble.